1. Field of the Invention
The invention relates to a spring-loaded brake for an actuator that drives a household screen of the roller shutter, blind, curtain, grille, projector screen or garage door type. This brake is more suited to motorized actuators and, more particularly, tubular motors. The invention also relates to a rotary actuator for driving a household screen fitted with such a brake.
2. Brief Description of the Related Art
The use of a helical spring-loaded brake in actuators for household screens is known, notably from patents FR-B-2 610 668 and U.S. Pat. No. 6,700,246. In those documents, a helical spring is mounted in a friction piece. At least one turn of the spring is radially stressed by a bore of the friction piece. Each end of the spring forms a lug extending radially towards the inside of the spring. Each lug can be moved in order to drive the rotation of the spring with respect to its axis. An action of an input piece on one side of the first lug causes the spring to rotate in a first direction. An action of an output piece on the opposite side of the first lug causes the spring to rotate in the second direction, that is to say in the opposite direction. Furthermore, the input piece may also act on the second lug of the spring in order to drive the rotation of the spring in the second direction. Likewise, the output piece may act on the second lug of the spring in order to cause the spring to rotate in the first direction. The input piece, the output piece and the spring are assembled in such a way that the action of the input piece on one of the two lugs of the spring tends to reduce the diameter of the external envelope of the spring. Thus, friction between the bore of the friction piece and the turns of the spring decreases, leading to a reduction in the radial stress between the spring and the friction piece. Conversely, action of the output piece on one of the two lugs of the spring tends to increase the diameter of the external envelope of the spring. Friction between the bore of the friction piece and the turns of the spring therefore increases. The same is true of the radial stress between the spring and the friction piece. As a result, the rotation of the input piece allows the spring and the output piece to rotate, while rotation of the output piece immobilizes the spring. In the latter case, the output piece becomes locked or, at the very least, is braked.
When the input piece acts on one lug of the spring, it causes the coil spring to rotate and releases the brake by reducing the friction between the turns and the bore of the friction piece. By rotating inside the bore, the spring moves axially because it behaves like a screw, the helix of the spring acting like the threads of the screw. Hence, when the input piece acts on a first lug, the spring moves axially in a first direction. Conversely, when the input piece acts on a second lug, the spring moves axially in a second direction opposite to the first.
In brakes described in the prior art, the helical spring is stopped axially on one side by the input piece and on the other side either by the output piece or by the friction piece. This design leads to asymmetric brake operation. Specifically, when the input piece drives the rotation of the spring, if the latter is pressed axially against the input piece, the brake operates correctly. By contrast, if the spring is pressed axially against the friction piece, the portion of the turn in contact with the friction piece rubs against this piece, giving rise to a parasitic resistive force which brakes the movement. Efficiency is therefore reduced. When the spring is pressed axially against the output piece a parasitic force is also observed during certain phases of operation of the brake. This phenomenon occurs when there is a variation in speed between the input piece and the output piece. This phenomenon is commonplace in the lowering of a screen, when the load is driving. These parasitic forces cause the system to vibrate, generating noise. Tests have shown that vibrations may arise, on lowering, for a brake torque which, when the spring is pressed against the output piece, is half that encountered in the configuration in which the spring is pressed against the input piece.